The present invention relates to a new and improved vulcanizable rubber mixture and to a new and improved vulcanized rubber material formed from such vulcanizable rubber mixture. The present invention also relates to a new and improved method of producing rubber articles from such vulcanizable rubber mixture.
Generally speaking, the present invention is concerned with a vulcanizable rubber mixture capable of forming, during vulcanization, a durable bond between such rubber mixture and copper alloys, specifically brass or brass-coated steel. For this reason, such vulcanizable rubber mixture hereinafter will also be called "bonding mixture or system".
The functional capabilities and the service life of rubber products containing metallic strength-producing or reinforcing support means which are imbedded into an elastomeric material, such as, for example, vehicle tires, industrial hoses or conveyor belts, are dependent to a large extent upon a durable bond formed between the strength producing or reinforcing support means and the vulcanized rubber mixture surrounding the same.
As such strength producing or reinforcing support means, there are used above all brass coated steel cords which generally are not specially prepared or pretreated for this purpose. Thus the bond must be directly formed between the brass coated steel cord surface and the rubber mixture which surrounds the same, during vulcanization. For this purpose, special bonding agents are admixed to the vulcanizable rubber mixtures or so-called bonding mixtures or systems. According to the prior art, there can be differentiated in principle two bonding mixtures or systems, namely "cobalt systems" and "resorcinol resin systems". Also, combined bonding mixtures or systems, i.e. combinations of cobalt systems and resorcinol resin systems are known and also have technical or industrial significance with respect to the bonding of steel cords.
Bonding mixtures or systems employing cobalt are rich in sulfur in most cases and contain cobalt in the form of organic salts, i.e. as cobalt soaps; particularly, cobalt octoate, cobalt stearate, cobalt naphthenate and cobalt resin soaps are used. In recent times there are increasingly also employed organic cobalt boron complexes which are believed to be advantageous with respect to aging.
The classical cobalt containing bonding mixtures or systems frequently require the addition of high amounts of sulfur so that the mixing and processing operations must be carried out in a very careful manner in order to prevent the efflorescence of sulfur. In comparison with the resin based bonding mixtures or systems, the cobalt containing bonding systems have, on the one hand, the advantage of higher corrosion resistance, however, on the other hand, the disadvantage of lower resistance against oxidative aging. Furthermore, the use of cobalt containing bonding mixtures or systems may be problematic in toxicological terms because cobalt containing dust is considered to cause cancer.
The typical classical resin based bonding mixture or system contains the constituents resorcinol, hexamethylenetetramine (HEXA) and active precipitated silicic acid. Hexamethylenetetramine which, if desired, can also be used as an alkaline accelerator, has the function of a methylene donor in this composition. Together with resorcinol, there is formed during vulcanization, a resorcinol resin which not only improves the bond but also increases the tensile stress values and the hardness of the vulcanized material.
There exists a plurality of possibilities for varying the resorcinol-hexamethylenetetramine-silicic acid bonding mixture or system; among others, there are mentioned here replacing resorcinol and hexamethylenetetramine by a resorcinol-hexamethylenetetramine 1:1 complex or a resorcinol-hexamethylenetetramine-boric acid 1:1:1 complex, replacing hexamethylenetetramine by other methylene donors, for example, hexamethylolmelamine hexamethyl ether (HMMM), and replacing resorcinol by resins on a resorcinol or resorcinol derivative basis which additionally may contain other phenols or triazine derivatives. The vulcanization rate of rubber mixtures containing such resin based bonding mixtures or systems is notably lower than the vulcanization rate of rubber mixtures containing resorcinol bonding mixtures or systems. In most cases this is also true when the proportion of the vulcanization retarding silicic acid is decreased.
Vulcanizable rubber mixtures produced using the classical bonding mixture or system resorcinol-hexamethylene-tetramine-silicic acid are superior to most other resin based bonding mixtures or systems with respect to mechanical properties and initial bonding. Above all, there can be attained high elasticity and good tensile stress values at good resistance against dynamically produced fractures or ruptures in the region of the strength producing or reinforcing support means. However, a grave disadvantage of such vulcanized materials resides in the low moisture resistance of the bonding to brass. This is also true for vulcanized materials which employ the various modified hexamethylenetetramine containing bonding systems. When employing hexamethylolmelamine hexamethyl ether instead of hexamethylenetetramine, there is obtained a significantly higher moisture resistance.
During processing of rubber mixtures containing free resorcinol, the free resorcinol is problematic in toxicological terms since, during admixture, the resorcinol is driven off with the formation of white pungent fumes. The methylene donors also are not toxicologically unobjectionable because they set free formaldehyde.